Influence of Al content on electrical resistivity and phase constitution of quenched β Ti-Mo-Al alloys

Abstract

Influence of Al content on electrical resistivity, hardness and phase transformation behavior of Ti-11 mass%Mo-(0 to approximately 7) mass%Al alloys quenched form 1323 K was investigated by means of resistivity and hardness measurements, X-ray diffraction analysis, and optical and electron microscopy. α″ martensite was observed between 1 and 5%Al and its volume fraction in Ti-11Mo-2Al alloy was the largest, which was estimated from the intensity of X-ray diffraction. On the other hand, no acicular microstructure or reflection of the α″ martensite was observed in 0Al and 7Al alloys. Vickers hardness decreased with increasing Al content up to 2%, reaching a minimum, abruptly increased up to 3 mass% Al, and gradually increased above 3%. Resistivity maximum on temperature-resistivity curve of 0Al alloy appeared at about 210 K. Negative temperature dependence was observed in 1Al, 3Al, 5Al and 7Al alloys, while 2Al alloy in which the volume fraction of α″ martensite was the largest had positive temperature dependence. From these results, it is considered that Ms, starting temperature of α″ martensitic transformation, was elevated with increasing Al content up to 2% and then lowered. Though temperature of β transus is slightly increased by Al content, the increase in Ms of alloys containing Al up to 2% will be attributable to decrease in resistance to lattice deformation, i.e. decrease in the degree of supercooling for martensitic transforming with increasing Al content up to 2% Al because of a hardness decrease due to decrease in the volume fraction of athermal ω that is larger than a hardness increase due to solution hardening of β phase with Al addition. Above 3%Al, it is considered that the decrease of Ms will be attributable to increase in the degree of supercooling because of the smaller effect of athermal ω than that of solution hardening.